Display screen or portion thereof with transitional graphical user interface



FIG. 1 is a front view of the first image in a sequence for a display screen or portion thereof with transitional graphical user interface, showing our new design; and,

FIG. 2 is the second image thereof.

The outer perimeter shown in dashed broken lines in the drawings illustrates a display screen or portion thereof that forms no part of the claimed design. The remaining dashed broken lines in the drawings illustrate portions of the transitional graphical user interface that form no part of the claimed design.

The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1-2. The process or period in which one image transitions into another image forms no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with transitional graphical user interface, as shown and described. 